The invention relates to voltage controlled oscillator (VCO) circuits in which the frequency of oscillation is a function of an applied voltage. Such oscillators find use in phase locked loop (PLL) systems where an oscillator is forced to oscillate at a frequency and phase that is related to an input signal. For IC applications the oscillator of choice is typically the well-known emitter-coupled multivibrator. This circuit employs all NPN transistors and is thus capable of operating at relatively high frequencies. This basic circuit is the subject of U.S. Pat. No. 3,857,110. An improved version is disclosed in Monticelli patent 4,468,636, which is assigned to the assignee of the present invention. The teaching in these two patents is incorporated herein by reference.
One of the remaining problems associated with the prior art VCO's is their limitation of a single input control voltage for controlling the operating frequency and phase. We have discovered that where a system includes more than one PLL and the individual loops must be coupled together, the VCO can perform this function if it has plural inputs. Desirably, these inputs are isolated and can enjoy predetermined voltage-to-frequency sensitivity so that system performance can be predetermined.